Several proteins involved in hemostasis are associated with platelets. Although for many years it was assumed that these substances were absorbed by circulating platelets from the plasma, recent work (Nachman, Levine, and Jaffe, J.C.I., 60:914-921, 1977) showed that factor VIII AG was synthesized by guinea pig megakaryocytes. Thus these proteins (factors V, VIII AG, XIII, fibrinogen, and fibronectin) are probably all intrinsic to platelets, implying a significant, although presently unknown, role for them in hemostasis and perhaps in thrombosis. We will use antibodies to these proteins to detect their synthesis by megakaryocytes and for ultrastructural localization in platelets and megakaryocytes.